This research will determine the efficacy of intramuscular (IM) recombinant human fibroblast interferon (rIFN-B) administered for exacerbating multiple sclerosis (MS) under randomized, double-blinded, placebo controlled conditions. Our previous double-blinded study demonstrated that intrathecal (IT) natural IFN-B was effective in reducing MS exacerbations. The IT route of administration for that study was dictated by the belief that systemic IFN- B did not cross the blood-brain barrier (BBB). However, there is new evidence indicating that systemic IFN-B does cross the BBB which compels assessment of systemically administered interferon in MS. This multicentered study will include 288 patients with active exacerbating MS who will be randomly assigned to receive IM injections of rIFN-B or placebo. Treatments will be performed weekly for 104 weeks. The primary measure of efficacy will be the intergroup difference in the time to progression in disability as measured by the Kurtzke Expanded Disability Scale of at least 1.0 units which persists for six months. Additionally, a number of other important parameters of MS disease activity and patient functional status will be analyzed as secondary outcome measures. Our recently completed Pilot Study determined that 6 x 106 IU administered IM weekly was the ideal dose and schedule of rIFN-B which significantly induced measures of interferon biologic activity under double-blinded conditions. Further at this dosage of rIFN-B, acetaminophen was effective in facilitating blinding, thus obviating the need for previously used indomethacin which had potentially confounding immunologic effects. If the results of this study are positive, then rFIN-B may be widely employed in the future as a practicle clinical too in the treatment of MS.